1. Field of the Invention
The present invention relates to thermoplastic polyurethanes having various excellent characteristics of high heat resistance, friction melt resistance, cold resistance and hydrolysis resistance with high compression set and having excellent melt-moldability, to molded articles comprising such thermoplastic polyurethanes, to resilient polyurethane fibers, to laminates composed of melt-molded layers of thermoplastic polyurethanes and fibrous base layers and to methods for producing such thermoplastic polyurethanes and resilient polyurethane fibers.
2. Background of the Invention
As having various excellent characteristics of high resilience, abrasion resistance and oil resistance, polyurethanes are specifically noted as substituents for rubber and plastics and have heretofore been widely used in various fields as molding materials to which ordinary methods of molding and machining plastics are applicable.
Known polyurethanes include polyether polyurethanes to be produced from polyether-polyols, polyester polyurethanes to be produced from polyester-polyols, and polycarbonate polyurethanes to be produced from polycarbonate-polyols. These polyurethanes are widely used as raw materials for fibers, sheets, films, adhesives, coating materials, etc. In general, polyether polyurethanes have excellent hydrolysis resistance but are poor in light fastness, aging resistance under heat and chlorine resistance. Polyester polyurethanes have excellent mechanical characteristics and abrasion resistance but are poor in hydrolysis resistance and mildew resistance. Polycarbonate polyurethanes have excellent durability in addition to the characteristics of polyester polyurethanes but are poor in cold resistance and are high-priced.
Conventional polyester polyurethanes which have been improved in the hydrolysis resistance and the flexibility at low temperatures are known. For example, Japanese Patent Laid-Open No. 61-185520 refers to polyester polyurethanes to be produced from polyester-polyols which have a molecular weight of from 500 to 30000 and which are obtained by reacting a mixture comprising 1,9-nonanediol and polyols of the following general formula (A) with dicarboxylic acids. EQU HO--(CH.sub.2)n-CR.sup.1 R.sup.2 --(CH.sub.2)n-OH (A)
wherein R.sup.1 represents a methyl group or an ethyl group; R.sup.2 represents a hydrogen atom, a methyl group, an ethyl group, a hydroxymethyl group or a hydroxyethyl group; and n represents an integer of from 1 to 5.
However, only polyurethanes as produced from polyester-diols having two hydroxyl groups in one molecule are concretely illustrated in the above-mentioned Japanese Patent Laid-Open No. 61-185520. It is hard to say that the characteristics of friction melt resistances heat resistance and compression set of these polyurethanes and molded articles comprising them are on the satisfactory level for practical use, although their hydrolysis resistance and flexibility at low temperatures have been somewhat improved over those of conventional polyester polyurethanes. In addition, if the polyurethanes are formed into resilient polyurethane fibers, their melt-spinning stability is poor so that the resilient polyurethane fibers formed are poorly homogeneous.
Methods for producing resilient polyurethane fibers are referred to in Japanese Patent Laid-Open No. 2-127515 and 4-11011, in which prepolymers comprising polyfunctional polyols and isocyanates are added to and mixed with molten polyurethanes and the resulting mixtures are then spun. According to the methods referred to in these laid-open patent applications, it is difficult to uniformly mix polyurethanes with prepolymers added thereto with the result that the compositions of the resulting polyurethane mixtures are uneven and therefore the fibers to be obtained by spinning the mixtures are poorly homogeneous. The polyester-polyols that are concretely disclosed in these laid-open patent applications as raw materials for polyurethanes all have a crystallization enthalpy (.DELTA.H) of more than 70 J/g, and these give resilient polyurethane fibers with poor cold resistance.